Microbial Desalination is a technology to remove salt ions from saline water (deionize saline water). Driven by electric current (electric potential difference) generated by the metabolic activity of exoelectrogenic microorganisms, i.e. driven by a bio-electric potential difference, ions are transported through ion selective membranes, thus generating an ion-enriched stream (brine) and an ion-depleted stream (desalinated water). Ion transport is effected in proportion to the electric current being generated by these microorganisms [1,2,3].

1. Technical Description
2. Plant Design
3. Feed Water
4. Desalinated Water & Brine
5. Data and Information
6. Preferred Use
7. Environmental Impact
8. Stage of Maturity
9. Further Developments
10. References

1. Technical Description

The Basics – Separation Principle and Operating Mechanism
An exoelectrogenic microorganism is a microorganism that has the ability to transfer electrons extracellularly. Exoelectrogenic microorganisms are also referred to as “exoelectrogens”, “electrochemically active bacteria”, “anode respiring bacteria” and “electricigens” [4]. A known species of exoelectrogens is, e.g., Geobacter sulfurreducens.

In a Microbial Desalination Cell (MDC) exoelectrogenic microorganisms serve to generate electric current (electrons) from oxidation of organic matter present, e.g., in wastewater [3] or in artificially created nutritious solutions. The electric potential difference between anode and cathode generated by the metabolic activity of these microorganisms is the driving force for desalination in an MDC. It is used to induce ion transport through ion exchange membranes [2,5], thus desalinating water.

Therefore, the driving force of desalination in MDC may be – more precisely – referred to as a “bio-electric” potential difference. The fundamental separation principle to separate ions from water in MDC is the capability of ions to move selectively according to their charge.

MDC is in nature similar to Electrodialysis (ED), yet different from ED because it does not require external electricity [1]. Instead, energy is provided by the exoelectrogenic microorganisms, being an integral part of the MDC.

It goes without saying that the particular attraction and benefit of MDC lies in the fact that MDC allows desalination without external electrical energy input and without the need of high feed pressure. In consideration of the relevance of energy cost in desalination, a desalination technology like MDC, which does not consume more energy than generated by itself, must be of particular interest.

A further benefit of Microbial Desalination Technology is that it combines at least two simultaneous functions: electrical energy production and desalination. Depending on design, it may also take up a third function. If, for example, wastewater is used to feed the MDC and the organics contained therein serve as food source for the microorganisms, the MDC can even achieve three functions simultaneously: energy production, desalination and wastewater treatment.

Process Description
The first proof of concept of MDC was demonstrated in 2009 by a 3-chamber configuration [1, 6, 7]. This basic Microbial Desalination Cell resembles a fuel cell, containing an anode and a cathode, between which a pair of ion selective membranes is inserted. Fig. 1 depicts the basic design of a Microbial Desalination Cell (MDC).

Fig. 1: MDC – basic design


It consists of three chambers:

1.    the anode chamber with an anion exchange membrane (AEM). Next to the anode a film of exoelectrogenic microorganisms is attached as a kind of biofilm. The chamber is typically filled with a liquid (e.g. wastewater) which contains nutrition for the exoelectrogenic microorganisms. These exoelectrogenic microorganisms oxidise organic matter, and, in doing so, release protons and electrons.
2.    the cathode chamber with a cation exchange membrane (CEM) positioned next to the cathode. This chamber, in which the electrons transferred from the anode are consumed by a chemical reduction reaction (as described below), is typically filled with a liquid that enhances cathode performance (e.g. water + air injection).
3.    the middle chamber between the two ion exchange membranes (AEM and CEM) is filled with feed water to be desalinated [1].

Process Description MDC
The MDC process (here with an air-cathode) is depicted in the following sequence (Fig. 2 A – C)

Fig. 2 A: MDC process - anode chamber     Fig. 2 B: MDC process - cathode chamber     Fig. 2 C: DC process - resulting stage

Exoelectrogenic microorganisms on the anode oxidise organic matter fed into the anode chamber. In doing so, they release protons (H⁺) and electrons (e^-) (see Fig. 2 A). The following equation represents the oxidation reaction at the anode [8]:

(CH₂O)_n + nH₂O → nCO₂ + 4 ne^- + 4nH⁺

The electrons (e^-) leave the anode chamber and are transferred as electric current via external wire to the cathode. Protons (H⁺) cannot leave the anode chamber, as they are rejected on the anion exchange membrane (AEM). The surplus of positive charge in the anode chamber resulting from protons (H⁺) produced, however, desires balancing. The only source of balancing charge is drawing negatively charged ions (Cl^-) through the AEM from the middle chamber into the anode chamber (see Fig. 2 A).
Simultaneously, in the cathode chamber, electrons (e^-) are received as electric current (see Fig. 2 B). There they combine, e.g., with the oxygen provided by an air-cathode or by oxygen injection into the cathode chamber. They combine as well with the protons (H⁺) drawn from the liquid in the cathode chamber (chemical reduction reaction) to form H₂O. The following equation represents the reduction reaction at the cathode [8]:

O₂ + 4 ne^- + 4nH⁺ → 2 H₂O

The extraction of protons required for the formation of H₂O leaves a deficit of positive charge in the cathode chamber which can only be replenished by attracting positively charged ions (Na⁺) from the middle chamber. As only positively charged ions can pass the cation exchange membrane (CEM), Na⁺ ions cross the CEM and replenish the protons consumed in the formation of H₂O (see Fig. 2 B).

As a result (see Fig. 2 C), the feed water in the middle chamber is depleted of salt ions, i.e. desalted. Thus, the generation of electric current through oxidation activity of exoelectrogenic microorganisms induces an electric potential difference between anode and cathode, which in turn induces a selective ion transfer through the AEM and CEM, thereby depleting the feed water stream from salt ions.

This depletion of ions in the middle chamber results in water desalination without any water pressurisation or use of draw solutions or externally supplied electrical energy. Instead, energy is generated while water is desalinated, and, as the case may be, organic matter is removed from wastewater while being metabolised by the microorganisms ́ activity in the anode chamber [1, 2, 5, 6, 8, 9, 10].

MDC Process Peculiarities
-    MDC operation does not require electrical energy input and no elevated pressure level like Reverse Osmosis (RO) [1].
-    MDC requires, however, specific exoelectrogenic microorganisms which themselves need a continuous supply of a nutritious substrate for their metabolic work. MDC also requires an aerobic environment on the cathode side, which may be achieved by oxygen injection or otherwise procuring an aerobic water environment in the cathode chamber [6,8].
-    The supply of microorganism substrate may be secured by connecting the MDC to a wastewater plant effluent to benefit additionally from wastewater treatment as a co-performing element of the MDC process.
-    MDC may constitute a (limited) barrier against bacteria, viruses and other microbes as the pore sizes of ion exchange membranes used will be in the range of 1 nm (about 350 Dalton), i.e. considerably smaller than the size of viruses and bacteria [2]. However, contradicting this opinion, it should be noted that not the complete volume flow to be desalinated needs to pass through the ion exchange membranes, thus limiting the true sanitary impact of MDC technology.
-    Due to yet missing field experience, it is not known to what extent MDC modules might need intermittent cleaning during operation or to what extent they might need to be preserved against biological growth in case of shut down for a prolonged period.

For further requirements concerning MDC plant layout, please also refer to Section 2 (Plant Design) hereinafter.


Motivation of Use
In consideration of the remarkable energy cost of other desalination technologies, MDC has been developed to overcome this drawback [1]. The idea to desalinate water without consumption of energy is certainly fascinating. It requires, however, the integration of another process element (energy production by exoelectrogens from digesting organic matter). At this point of time, MDC has a unique potential to emerge as a technology to perform three fundamental tasks simultaneously: (seawater) desalination, electric energy generation and wastewater treatment [8].


Technical Variations
MDC Design Variation and Optimisation
The basic MDC cell, incorporating three chambers forming one stack, is illustrated in Fig. 1 above.

While this type of model will be indispensable for lab purposes, multiple stack designs are desirable to increase desalination volume and to decrease manufacturing cost. Multiple stack structuring is achieved by repetitive addition of AEM and CEM membrane pairs to the module as illustrated in Fig. 3 below, thus creating repeating desalting and concentration cells [7].

Fig. 3: Stack structure of MDC

Stacking increases the magnitude of ionic separation. In a stacked MDC, a single electron transfer from anode to cathode can separate as many ion pairs as the number of repeated AEM and CEM membrane pairs. For instance, in a 3 cell pair stack, a single electron transfer separates 3 pairs or mono-valent cations and anions by use of an (ideal perm-selective) ion exchange membranes [2,8].

Other basic design considerations in MDC technology relate to:
a) sizing of the anode surface in relation to cathode surface, whereby the cathode appears to require larger surfaces or a boost in speed of performance due to the relatively low speed of chemical reduction performance of the cathode;
b) sizing of the anode chamber volume versus the desalination chamber and cathode chamber volume. It appears that anode chambers require a significantly larger volume than the other chambers;
c) choosing optimum electrode materials, particularly for the cathode due to its slow chemical conversion characteristics. In this context, improved material choice for the purpose of elevated current densities per m² of electrode surface is of particular interest [11];
d) choosing optimal electrode materials to improve electrode stability and lifetime.


MDC Operational Variations & Performance Optimisation
As MDC works with organic matter and may fulfil three tasks at once, there are numerous parameters to work on simultaneously. Research is on its way to study variations of parameters and analyse patterns of dependence. Yet, as MDC technology is relatively young, most current knowledge is based on lab tests. To the best of the author ́s knowledge, MDC field experience is still missing. Therefore, the following aspects of operation are of indicative nature and observations may yet not all be fully studied. Particular complexities are apparent when applying MDC:
-    A particular optimisation complexity of MDC is to understand the convergence or divergence of achieving three optimisation targets simultaneously, i.e. maximum electrical power generation, highest desalination volume or highest degree of desalination and highest reduction of organic matter.
-    A further particular complexity of MDC is to work with a multitude of parameters that may be altered and to understand their relative alteration contribution on the desired performance targets, both technically and commercially.

Some operational aspects, however, have been researched already in some detail and appear to have influence on MDC performance and its optimisation:
-    pH value variation between anode and cathode chamber: As the desalination cycles proceed, the pH value in the anode chamber decreases due to proton accumulation by microbial respiration and increases in the cathode chamber due to hydroxide accumulation by oxygen reduction [8]. Therefore, decrease in pH value is a major challenge for MDC operation [2]. It adversely affects both, the microbial activity and growth in the anode chamber and the chemical reduction efficiency of electron acceptors (O₂) in the cathode chamber. Such pH imbalance may be alleviated by increasing the anodic chamber volume or by adding acid or base. The addition of acid or base as well as the addition of a phosphorous buffer solution do not appear feasible with respect to cost and environmental concerns. Furthermore, frequent exchange of the anolyte solution, i.e. the fluid contained in the anode chamber, has proven beneficial [6].
A recirculation of a fraction of the anolyte to the cathode chamber and from there back to the anode chamber, a so-called recirculation microbial cell (rMDC) design has been investigated to overcome the pH problem. As this solution generates new issues (e.g. transfer of O₂ to the anode side), an economic and sustainable method of controlling the pH remains an urgent development need in MDC technology [6, 8].
-    Impact of Internal (Ohmic) Resistance: A decrease in conductivity of the saline feed water increases the internal resistance (IR), i.e. the resistance within the MDC cell and thus decreases the overall electric cell potential of the MDC. As a countermeasure, membrane distance may be increased to decrease IR [1, 6].

Also, stacking of MDC improves the desalination rate, but also leads to a higher electric potential loss due to an increase in IR from increased stacking [2, 6].

MDCs operated with lower Hydraulic Retention Time (HRT), i.e. the period until exchange of the anolyte, shows lower IR [6].
-    Impact of External (Ohmic) Resistance:
     The external resistance (ER), i.e. resistance in the wire between anode and cathode, is of importance for the desalination efficiency. At lower ER values (e.g. 10 Ohm instead of 100 Ohm), the electrical current density (in theory) is higher and thus increases the rate of desalination. In other words, more Total Dissolved Solids (TDS) is removed with lower ER (i.e. higher electrical current density) [6].
-    Batch vs. continuous MDC operation:
     In batch mode, the performance of MDCs decreases with time as the internal resistance (IR) increases due to the lowering of electrolyte conductivity in the desalination chamber. Cyclic batch mode, or even better, continuous mode operation can be the optimum choice for practical operations because it reduces total capital cost and facilitates control of decisive parameters such as pH and substrate concentration [6].
-    Variation of Hydraulic Retention Time (HRT):
     MDCs operated with lower HRT of anolyte solution exhibit lower Internal Resistance (IR) [6, 12].
-    Scaling of CEM membranes and fouling of AEM membranes:
     Depending on the salinity of the water to be desalinated, CEM scaling is of significant importance for MDC performance [6, 7]. A complete prevention of membrane scaling appears impossible; however, use of an oxygen cathode with acidified water may alleviate scaling [7]. Membrane fouling may be observed on AEM due to biofilm growth [5, 6]. Chosen ion exchange membranes also differ considerably in pore size, degree of organic matter.
-    Choice of exoelectronic microorganisms:
     It will be obvious that the metabolism of microorganisms differs from species to species. Therefore, study of MDC performance in terms of electron generation capabilities and degradation potential of organic matter are relevant areas of study [13].

Differences to and Similarities with other Relevant Technologies
The MDC process has great similarity with Electrodialysis (ED), but in the case of ED external electrical energy is used to provide energy for the separation of ionic species.
With respect to Reverse Osmosis (RO), MDC does not need pressure elevation to high levels.
However, different to all other desalination technologies, MDC integrates bio-electrochemical energy production with desalination and water treatment, i.e. performs multiple functions. Due to this specialty, a one to one comparison of MDC to other (pure) desalination technologies is not advisable, also because the technologies have different outputs.

A further comparison of benefits and drawbacks vis à vis other desalination technologies appears premature due to the fact that MDC does not yet have practical field experience for realistic comparison.

2. Plant Design

Process Requirements
A full plant arrangement may - apart from the core desalination unit (MDC) - also require the following:
-    A feed pump for supplying pretreatment and desalination unit with feed water is needed.
-    Appropriate pretreatment to limit scaling and fouling of MDC desalination chambers with elevated scaling risk seems to be necessary.
-    Appropriate process treatment to reduce scaling potential, particularly on CEM membranes, appears necessary.
-    Depending on finally preferred channel width of the desalination chamber (e.g. 0.3 – 1.5 cm) and the preferential widths of anode and cathode chambers, the MDC cell requires some specific degree of protection against blocking by particles which may differ for the three chambers into which fluids are fed. Particle filters in the range of a few microns could serve as the last step of pretreatment for the feed water to be desalinated, for the influent water from waste water treatment process to be fed to the anode chamber as well as for the liquid in the cathode chamber [6].
-    Depending on application, the feed water streams may need further individual pretreatment to protect the micron filtration systems suggested beforehand (e.g. further pre-filtration step at larger mesh size).
-    Depending on Total Suspended Solids and Total Organic Carbon (TSS/TOC) loading of the feed water, further pretreatment to reduce TSS/TOC volume may be required for the feed water to be desalinated; also, the TSS/TOC levels of the anolyte and catholyte need separate specification.
-    In applications with elevated scaling risk, appropriate process treatment to reduce scaling potential, particularly on CEM membranes, appears necessary.
-    Compressed air injection may be required for optimised cathode performance.
-   Cleaning-in-Place (CIP) unit (possibly with neutralisation) for cleaning of MDC modules may be a requirement.
-    A wastewater disposal facility to allow for occasional CIP wastewater discharge (if in fact required), intermittent filter backwash water discharge and sludge discharge for used up anolyte discharge may be needed.
-    Depending on application, posttreatment of the desalinated water may be required.

MDC Plant Layout
Due to the advantage of being able to build MDC units module based, MDC plants are scalable in size [3]. Fig. 4 illustrates an exemplary MDC system arrangement.

Fig. 4: Exemplary MDC system arrangement (in conjunction with wastewater treatment plant)

3. Feed Water

1.    Different from other desalination technologies, three feed streams need to be considered: the feed flow of the water to be desalinated, the wastewater treatment flow connecting to the anode chamber and the water flow feeding the cathode chamber.
2.    Without any field experience so far, it is premature to take a judgement on the potential techno-commercial limitations of MDC due to the level of feed water salinity of the water to be desalinated.
3.    As far as other pretreatment requirements are concerned, please refer to the points mentioned hereinabove.
4.    Temperatures may be limited to AEM and CEM operational limits, possibly also to anode and cathode temperature limitations.
5.    For general guidance purpose, the following Table 1 characterises general MDC feed water limits for the water to be desalinated.

Table 1: Stage of development of MDC technology in relation to water salinity [2, 6]

Because of the comparatively low desalination efficiency for high-salinity water, one potential application of MDC technology could be brackish water desalination [6].

4.    Desalinated Water & Brine

Without any field experience so far, it is premature to characterise desalinated water and discharge flows in volume or quality other than recognising their existence as depicted in Fig. 4 above.

5.    Data and Information

In the absence of field test results so far, it does not appear appropriate to take assumptions on data and information under this chapter.

6.    Preferred Use

The techno-commercial feasibility of MDC has not yet been demonstrated in the field. Therefore, the preferred uses listed below are of indicative nature only:
1.    Seawater desalination
       Lab tests vary widely < [1,9] but show up to 99 % salinity removal depending on Hydraulic Retention Time (HRT) [6].
2.    Brackish water desalination
       An obstacle of low salinity desalination by MDC is that the low conductivity increases the internal resistance on the MDC. This disadvantage may be overcome by adding an ion exchange resin to the desalination chamber. Also, salt removal from the desalination channel increases the salinity of the anolyte which may negatively affect the activity level of the microbial community in the anode chamber [6].
3.    Water softening
       Up to 90 % hardness removal has been demonstrated in lab tests [6].
4.    Ground water remediation (in particular nitrate removal) may be a feasible application [6].
5.    Simultaneous desalination, wastewater treatment and hydrogen production (while adding energy to an MDC process in a modified “Microbial Electrolysis and Desalination Cell”) may be a desired application [6].

7.    Environmental Impact

With respect to energy consumption the MDC may be considered a sustainable process, as it is based on a renewable primary energy source. However, research has not yet demonstrated dependable field test results to judge environmental impacts of MDC professionally.

Also, and as mentioned above, it appears likely from lab test demonstrations that MDC will not be able to survive without countering measures of scaling and fouling. In this respect dosing of chemical additives is very likely to be required to maintain MDC performance.

8.    Stage of Maturity

Table 2 summarises MDC technology with respect to the technology ́s stage of maturity.

Table 2: Stage of maturity of MDC technology

9.    Further Developments

These are the challenges for improvement in MDC:
1. Primary target for MDC will be to reach field test level to measure and benchmark real performance and to generate cost reduction from exploiting scale effects [14].
2. Scaling up of MDC devices will be a further target. While primary tests have been performed with desalination chambers of 3 ml, larger desalination chambers (15 litres) are subject of more recent investigation. Simultaneously the anode chamber has been increased from 27 ml to 60 litres. The largest MDC (total volume of 105 litres) has achieved an electricity generation of 2,000 mA and a salt removal rate of 9.2 kg/m³/d [6, 14].
3. For developing more breadth and depth in applications, it is envisioned that MDC may be used as pretreatment of RO systems to reduce the overall cost of desalination [6]. In particular, the energy produced in MDC operation could be used to run downstream RO or Electrodeionisation (EDI) plants.
4. Major challenges for the development of MDC as a desalination technology are:
   - managing pH variations;
   - managing scaling and fouling, in particular cathode scaling;
   - controlling internal and external resistance;
   - improving anode and cathode materials and longevity;
   - improving exoelectrogenic bacteria performance.

10. References

[1] Cao, Xiaoxin; Huang, Xia; Liang, Peng; Xiao, Kang; Zhou, Yingjun; Zhang, Xiaoyuan; Logan, Bruce E.: A New Method of Water Desalination Using Microbial Desalination Cells. Environmental Science & Technology 43 (2009), pp. 7148-7152.
[2] Kim, Younggy; Logan, Bruce E.: Microbial desalination cells for energy production and desalination. Desalination 308 (2013), pp. 122-130.
[3] Subramani, Arun; Jacangelo, Koseph G.: Emerging desalination technologies for water treatment: A critical review. Water Research 75 (2015), pp. 164-187.
[4] Logan, Bruce: Exoelectrogenic bacteria that power fuel cells. Nature Reviews Microbiology 7 (2009), pp. 375-383.
[5] Wang, Heming; Ren, Zhiong Jason: A comprehensive review of microbial electrochemical systems as a platform technology. Biotechnology Advancs 31 (2013), pp. 1796-1807.
[6] Sevda, Surajbhan; Yuan, Heyang; He, Zhen; Abu-Reesh, Ibrahim M: Microbial desalination cells as a versatile technology: Functions, optimization and prospective. Desalination 371 (2015), pp. 9-17.
[7] Barastad, Kristen S.; He, Zhen: Water softening using microbial desalination cell technology. Desalination 309 (2013), pp. 32-37.
[8] Saeed, Henna M., Husseini, Ghaleb A.; Yousef, Sharifeh; Saif, Jawaria; Al- Asheh, Sameer; Abu Fara, Abdullah; Azzam, Sara; Khawaga, Rehab; Aidan, Ahmed. Microbial desalination cell technology: A review and a case study. Desalination 359 (2015), pp. 1-13.
[9] Luo, Haiping; Xu, Pei; Roane, Timberley M., Jenkins, Peter E., Ren, Zhiyong: Microbial desalination cells for improved performance in wastewater treatment, electricity production, and desalination. Biosource Technology 105 (2012), pp. 60-66.
[10] Asadi-Ghalhari; Mehrdadi, Nassar; Nabi-Bidhendi, Gholamreza: Simultaneous Desalination of Was water and Electricity Production with New Membrane Technolgoy, Air-Cathode Microbial Desalination Cells. Current World Environment, Vol. 10(1) (2015), pp. 115-120.
[11] Sharma, Mohita; Bajracharya, Suman; Gildemyn, Sylvia; Patil, Sunil A.; Alvarez-Gallego, Yolanda; Pant, Deepak; Rabaey, Korneel; Dominguez-Benetton, Xochtil: A critical revisit of the key parameters used to describe microbial electrochemical systems. Electrochimica Acta 140 (2014), pp. 191- 208.
[12] Qu, Youpeng; Feng, Yujie; Liu, Jia; He, Weihua; Shi, Xinxin; Yang, Qiao; Lv, Yiangwei; Logan, Bruce: Salt removal using multiple microbial desalination cells under continuous flow conditions. Desalination 317 (2013), pp. 17-22.
[13] Logan, Bruce: Bioenergy production using microbial fuel cells. Presentation posted on YouTube Jan. 6, 2016. URL: https://youtube/x-iN-O89TNA (Stand 21.08.2017)
[14] Zhang, Fei; He, Zhen: Scaling up microbial desalination cell system with a post-aerobic process for simultaneous wastewater treatment and seawater desalination. Desalination 360 (2015), pp. 28-34.

Capacitive Deionisation (CDI) is a technology for removing salt ions from water by applying an electrical voltage difference between two porous carbon electrodes in which the ions will be temporarily stored [1].

The driving force of the process is the application of an electrical voltage difference [2]; the separation principle of salts from water is based on the capability of ions to move selectively according to their charge; in different words: positively and negatively charged salt ions in aqueous solutions migrate towards electrodes of opposite charge.

1. Technical Description
2. Plant Design
3. Feed Water
4. Desalinated Water & Brine
5. Data and Information
6. Preferred Use
7. Environmental Impact
8. Stage of Maturity
9. Further Developments
10. References

1. Technical Description

The Basics – Operating Mechanism

CDI´s preferred use is in low-salinity desalination applications (in principle below 10,000 ppm [1], in practice below 4,000 ppm [3]), where the benefit is lower energy consumption and higher recovery rates in comparison to Reverse Osmosis (RO) or distillation as alternative methods of brackish water desalination. This is mainly because CDI removes only salt ions, i.e. a tiny mass, from the saline feed water, whereas in RO or distillation a considerably larger mass of water is removed from the saline feed water [1].

Literature has traditionally classified CDI by its nature as an electrochemical technology, however, recent research [1] suggests that the most fundamental underlying principle of CDI is capacitive ion storage, a phenomenon based on the formation of a so called electrical double layer (EDL). In simple words: upon applying an electrical charge, ions are captured electrostatically and stored capacitively (analogous to electrical charge being stored in an electrical condenser/“capacitor“) in a diffuse layer formed next to the carbon surface. Ideally, pure physical accumulation of ions (sorption) dominates the process, not chemical (faradaic) reactions. The process is therefore easily reversible. Congruently, CDI may also be viewed as a process of ion sorption and desorption on electrically charged surfaces; CDI is, therefore, also named Electrosorption [1].

The formation of a capacitive EDL has today been identified as the actually dominant underlying principle for ion removal and is, therefore, at the heart of in-depth understanding and optimising the CDI process and its performance [1].

Apart from the aforementioned “capacitive ion storage“, a number of further electrochemical mechanisms are believed to accompany the “capacitive ion storage” phenomenon in a CDI cell which may influence CDI performance. Abstracting from the presence of such parallel electrochemical side effects, the CDI process – in its ideal form - is of purely physical nature and potentially enables CDI devices to have a long service life and low maintenance [1].

In practice, however, some electrochemical impacts are undoubtedly active, typically leading to some degree of electrochemically induced deviation from ideal performance of CDI cells over time (mainly faradaic reactions) [1].

Following the above explanations, CDI is a desalination technology that operates by adsorbing ions in an EDL formed in the proximity of electrodes by the application of an electrical potential difference. These electrodes are made of porous carbons optimised for high (ion) salt storage capacity and for good ion and electron transport [1].

In a nutshell, two fundamental technical phenomena need to be mastered and controlled in CDI systems [1]:

1. “capacitive ion storage“ (capturing ions electrostatically and storing them capacitively)
2. “ion kinetics“ (the differences in mobility of the various ions on EDL formation).

While scientists may have today opposing convictions about the most dominantly underlying principle of CDI being of physical or chemical nature, categorisation also appears to depend on the breadth of definition of the term “electrochemistry”. It will be an interesting debate of whether or not it is preferable to hold on to historic practice of categorising CDI methodologically under the class of electrochemical desalination technologies or differently.

Process Description CDI

The basic mechanism underlying CDI is schematically shown in Fig. 1. It depicts a single CDI cell. In practice, multiple CDI cells are stacked on top of each other to form a stack; several stacks are packed together to form a CDI module. The process of CDI comprises three steps:

Step 1 – Purification (Ion Electrosorption) (see Fig. 1 A)

Upon applying a direct current (DC) voltage difference (typically between 1.2 and 1.5 V) between two porous carbon electrodes (typically attached to metallic current collectors) ions are attracted to the respective electrodes, i.e. cations (ions with positive charge, here, for example, Na⁺) to the negatively charged electrode, and anions (ions with negative charge, here, for example, Cl^-) to the positively charged electrode. During this first “ion electrosorption” step, ions are stored in each of the two porous carbon electrodes. As a result, the saline feed water gets more and more deprived of the salt ions and, thus, is desalinated [1].

Fig. 1 A: CDI purification step     Fig. 1 B: CDI regeneration step     Fig. 1 C: CDI flushing step

Step 2 – Regeneration Step (Ion Desorption) (see Fig. 1 B)

As the electrodes become saturated with ions electrosorbed into the porous carbon, the electrodes require regeneration after some time. For regeneration purpose, the potential difference is reduced to zero (or reversed) and, in consequence, ions are desorbed from the electrodes and released back into the water stream [1] for the purpose of subsequent flushing them out with the bulk fluid as brine.

Step 3 – Flushing Step (see Fig. 1 C)

Finally, the desorbed ions are released into the water stream and flushed out of the CDI cell as brine. A further desalination cycle can restart hereafter [3].

To separate desalinated water from intermittent brine flushing, corresponding flow separation needs to be procured in the system´s piping arrangement.

A disadvantage of this CDI design is that – when reversing the electrical potential difference – ions may be driven to the electrode on the opposite side of the feed water channel. Once the electric potential difference is reversed, the adsorbed ions will be desorbed; at the same time, however, oppositely charged ions from the bulk fluid will be attracted and adsorbed. Therefore, ion desorption and adsorption occur simultaneously during this step. Consequently, some typical drawbacks will need to be taken into consideration, namely:
– incomplete regeneration of the electrodes leading to a deterioration in their adsorption capacity,
– longer regeneration times and
– residual ion accumulation blocking the way of other ions during the next purification step [4].

To avoid this drawback, a modification of CDI exists using specific membranes which is generally referred to as “Membrane Capacitive Deionisation” (MCDI).

Process Desciption MCDI

To enhance desalination performance, traditional CDI cells are modified by integration of ion selective membranes, as depicted in Fig. 2 A. This design is known as Membrane Capacitive Deionisation (MCDI) (see Fig. 2 B).

                     Fig. 2 A: CDI basic design                                                                                               Fig. 2 B: MCDI basic design

In MCDI, ion selective membranes (ion exchange membranes) are applied as thin films (typically 50 – 200 μm thickness) or coated (typically 20 μm thickness) directly to each of the two carbon electrodes, i.e. a cation exchange membrane to the cathode and an anion exchange membrane to the anode [1]. Cations can pass cation exchange membranes freely, anions are repelled; vice versa, anions can move freely through anion exchange membranes, cations cannot.

The MCDI process works similarly to the CDI process, as shown in Fig. 3.

Fig. 3 A: MCDI purification step     Fig. 3 B: MCDI regeneration step     Fig. 3 C: MCDI flushing Step

As a particular advantage of MCDI, the ion selective membranes attached to the respective electrodes hinder salt ions to get electrosorbed by the opposite electrode when switching the potential difference during the regeneration step [4].

CDI Process Peculiarities

1. Sequential production of desalinated water is a peculiarity of (M)CDI operation. In some typical CDI cell geometries (M)CDI desalination water production is intermittently interrupted by the need to flush the (M)CDI cell. Precise valve switching for the separation of brine from desalinated water flow is, therefore, essential.
2. Energy requirements of (M)CDI strongly correlate with the level of feed water salinity and to the level of ions removed from the feed water [2].
3. (M)CDI does not provide a barrier against bacteria, viruses and other microbes [5].
4. (M)CDI requires prevention against scaling and fouling [6].
5. Pressurised air is required when (M)CDI modules use air scouring [7]. The latter is a method to remove sediments and to avoid biofilm build-up. In air scouring, clean (compressed) air is injected into the feed flow of the (M)CDI cell, thereby generating a series of air slugs. The slugs scour away the sediments while the water flow expels the undesired waste to an outlet to be discharged.
6. (M)CDI systems are module based; (M)CDI plants are, therefore, scalable in size.
7. (M)CDI modules need to be preserved against biological growth by a biocide conservation solution in case of shut-down for a prolonged period, e.g. a week or more [8].

Requirements pertaining to the layout of an (M)CDI plant are covered under section “Plant Design“ hereinafter.

Motivation of Use

(M)CDI has been developed as a non-polluting, energy-efficient and cost-effective alternative to desalination technologies such as RO and Electrodialysis (ED) [9].

Technical Variations

1. Basic CDI cell design (CDI or MCDI)
   As mentioned in section 1, CDI cells are used in the traditional CDI design or in membrane enhanced MCDI design. MCDI design has operational advantages.
2. Basic CDI cell geometries
   CDI cells may be built in different geometries. However, the so-called “flow-by” mode in which the water channel lies in-between the electrode pair, as illustrated in Fig. 1, represents the very typical cell geometry today. The pathway of the feed water flows between the electrodes can be designed as an open channel (about 1 mm thickness) or may be constructed from spacer material, then about 100 - 300 μm in thickness. Flow may be one-dimensional from an entry point in a straight line to the exit point, as depicted in Fig. 1 hereinabove or e.g. in a 2-dimensional plane from corner to corner or from a central input with radial flow [1].
3. Increased independence from feed water salinity fluctuation
   For making the CDI process less sensitive to variations in the feed water salinity, a recycle line with reservoir to take up desalinated water being recycled may be worthwhile considering [2]. (→ see Fig. 4).
4. Electrical operation mode of CDI plant
   a) Constant voltage operation
   In constant voltage operation, the effluent salt concentration increases first, but then decreases over time, as electrodes will get saturated with ions over time. In other words: in the beginning, the EDLs are still uncharged and, hence, the potential difference between electrodes (the driving force of ion electrosorption) is high. Over time, with more and more ions becoming electrosorbed in the EDLs, the EDL potential (charge) increases and the remaining potential difference between the electrode pair, which drives the ion transport, decreases. Because of the decreasing ion removal rate over time, the effluent concentration increases again [1].
   b) Constant current operation (MCDI design)
   As the ionic charge transported into the porous electrodes is equal to the applied electric current, applying a constant current allows a better control of the effluent salt concentration compared to constant voltage operation. For stable effluent salt concentration, an MCDI design (i.e. incorporation of ion selective membranes into the CDI cell) is preferable [1]

Differences to and Similarities with Other Relevant Technologies

(M)CDI appears to compare favorably to RO on operational cost [1] and has come closer in recent years to get to parity with comparable RO investment amounts [10]. CDI compares favorably to RO on energy cost and environmental burden [10]. In particular the higher recovery rates of CDI compared to RO decrease water intake and disposal volume and add to the sustainability benefit of CDI versus RO [11]. However, unlike RO, (M)CDI does not provide a barrier against microbes.

Compared to Ion Exchange (IX) in e.g. softening applications, (M)CDI, like RO and other electrochemical desalination technologies, is not competitive on the amount of investment, but extremely competitive on operational and environmental cost [7].

Compared to electrochemical desalination technologies such as Electrodialysis/Electrodialysis Reversal (EDR) [11] and Electrodeionisation (EDI) and Continous Electrodeionisation (CEDI) [12], relative differences are less clearly documented in literature and may need to be evaluated case by case. All such technologies, however, operate at high recovery rates, whereby EDI/CEDI appears to be limited even to brackish total dissolved solids (TDS) levels [2], but then again can produce water with extremely low salinity (e.g. boiler feed water or high purity water), which is not the domain of CDI.

2. Plant design

Process Requirements

Besides the core desalination system (M)CDI, a full plant arrangement reguires the following components:

• feed pump (> 3 bar head pressure) for conveying the feed water through pretreatment and desalination unit
• appropriate pretreatment, particularly in applications with elevated scaling risk, to avoid scaling and fouling of (M)CDI cells, e.g. pH adjustment, anti-scalant dosing etc. [6]
• a protection for the (M)CDI cell against blocking by particles given the narrow spacer channels of the (M)CDI cell; typically, a 5 μm particle filter will do as the last step of pretreatment [7]
• further pretreatment to protect the 5 μm filtration unit (e.g. further pre-filtration step at larger mesh size) depending on application
• further pretreatment to reduce the volume of total suspended solids (TSS) and total organic carbon (TOC) depending on TSS/TOC loading of the feed water
• pressurized air (6 bar), if air scouring of (M)CDI units is desired for enhanced performance [7]
• a cleaning in place (CIP) unit (possibly with neutralisation) for intermittent acidic cleaning of CDI modules [10]
• a waste water disposal facility to allow for occasional CIP waste water discharge and intermittent flushing water discharge
• no particular post-treatment other than mandated by the application of the desalination system
• voltage, electrical current and TDS control to monitor and control system operation; in particular, precision of valve switching to separate concentrate flow from desalinated water flow for a high recovery rate
  

(M)CDI Plant Layout

Due to (M)CDI being module-based, (M)CDI plants are scalable in size [7]. Fig. 4 shows a generic (M)CDI plant layout in batch mode operation, and, alternatively, in continuous mode operation (dotted line with recycle loop).

In batch mode operation the product water from a first pass through the (M)CDI desalination unit is collected and may then reinjected again and again as required for a desired level of desalination. As such batch operation is tedious and costly, a continuous mode of operation is generally preferred. This is achieved by recycling the product water via a recycle loop. In continuous mode operation the degree of desalination can be easily pre-set and variations in feed water salinity are more easily controllable.

3. Feed Water

1. Different from RO and distillation technologies, energy consumption of (M)CDI is strongly related to salinity [5].
2. (M)CDI performs well in brackish water applications, preferably at a feed water salinity below 4,000 ppm TDS [7].
3. To avoid blockage of the spacer channel in CDI modules, particles in the feed water stream need to be controlled. Typically, a 5 μm pre-filtration will be appropriate [8/6].
4. Temperatures may be slightly more elevated than in RO (< 60° C) [6].
5. Feed water scaling potential needs to be evaluated [6].
6. For general guidance, Table 1 characterises general (M)CDI feed water limits [6].

* Limits depend on set TDS reduction and water recovery

Table 1: Typical feed water composition for (M)CDI [6]

Table 2 illustrates existing and future applicability of (M)CDI technology in relation to the water salinity in which the technology is applicable. While literature sees the brackish water salinity segment (1,000 – 10,000 ppm salinity) as the core applicational segment for (M)CDI technology today [3, 6, 7, 8], there are meanwhile also claims from (M)CDI suppliers to have supplied first (M)CDI systems to the salinity segment above 10,000 ppm, even to applications above 100,000 ppm [13]. However, these applications have not been typical seawater desalination applications, but mainly were used to concentrate produced water in the oil & gas industry. Results on techno-commercial feasibility are not yet in the public domain.

Table 2: Stage of development of (M)CDI technology in relation to water salinity [3, 6, 7, 8]

4. Desalinated Water & Brine

The level of remaining salts in deionised water from a (M)CDI desalination process is highly dependent on the recovery rate applied and the energy input. The level of remaining ions in the desalinated water is – in limits – adjustable in (M)CDI operation [14].

Typically, (M)CDI produces deionised water at high recovery rates compared to RO [11]. As a result, the volume of brine in (M)CDI systems is generally lower than in RO systems. Depending on application (e.g. antiscalant dosing, pH adjustment), the brine of (M)CDI systems may or may not be contaminated by chemical additives.

Depending on application, waste streams will come from disposing intermittent CIP (cleaning-in-place). Due to (M)CDI´s capability of deionising at comparatively high recovery rates, feed water volume and brine disposal volume will usually be lower than in RO operation [11].

5. Data and Information [6]

1. Typical field of economical desalination with regard to energy consumption:
   Brackish water < 4,000 ppm TDS
   
2. Feed water temperature 5 – 60 °C 
3. Typically, higher recovery rate than RO (up to 95 %)
4. Low pressure application, e.g. feed pressure < 3 bar [4]
5. Compressed air requirement (6 bar) for air scouring
   
6. Typical CDI cell voltage 1.2 – 1.5 V
7. CDI specific energy consumption strongly depends on feed water salinity, as shown in Fig. 5.
8. The specific energy consumption of CDI for feed water below TDS 4,000 ppm is significantly lower than that of RO, as depicted in Fig. 5.
9. Exemplary module and system data [4]
   • 1 module contains 12 CDI stacks
   • 1 stack contains 23 CDI cells (flow between 2 electrodes)
   • smallest system: 2 modules, total flow rate: 0.2 – 1.8 m³/h -
   • system at industrial scale: 12 modules, total flow rate 1.4 – 10.8 m³/h
   • typical module flow rate: 0.4 m³/h at 80 % recovery rate
10. CDI is operated fully automatically (low personnel cost for operation)
11. Module replacement (no reliable data available yet)
    
    

6. Preferred Use

1. Brackish water desalination (energy efficient below 4,000 ppm feed water TDS) [7]
2. Water softening
3. Commercial laundry waste water deionisation (waters with temperatures too high for RO, but manageable for CDI)
4. Treatment of cooling tower make-up water (side stream filtration; IX replacement)
5. Waste water / RO brine recovery
6. Horticulture irrigation (maintaining of tuneable ion levels in the desalinated water) [14]

7. Environmental Impact

With energy consumption typically below RO in comparable brackish water applications [10], CDI ranks on a privileged level of sustainability in brackish water desalination.

Like RO systems, also (M)CDI systems are prone to inorganic and organic fouling in practical applications, even though it has been shown that switching the potentials of electrodes, as is standard in CDI operation, may drastically reduce fouling [2]. Also, (M)CDI is a low pressure operation compared to RO, which in principle should reduce fouling tendency.

Dosing of chemical additives in CDI operation is typically restricted to intermittent CIP (cleaning-in-place) with acids, and – as may be required – to antiscalant and/or pH adjustment in case of elevated scaling potential [6].

While (M)CDI systems appear to be affected by fouling in practical operation, these fouling problems may in tendency be viewed as somewhat less severe compared to RO [2].

Depending on application, (M)CDI brines will be or will not be contaminated by chemical additives. Depending on the application, CIP waste water requires neutralisation and disposal. Due to (M)CDI´s capability of deionising at comparatively high recovery rates, feed water utilisation and brine disposal will be lower than in RO operation
[2].

In comparison to RO, (M)CDI may in total be ranked preferable in environmental sustainability.

8. Stage of Maturity

Table 3 summarises (M)CDI technology with respect to the technology´s stage of maturity.

 Table 3: Stage of maturity of (M)CDI technology

9. Further Developments

Challenges for improvement in CDI are:

1. To develop sufficient application breadth and depth for CDI, so as to lower CDI investment from experience and volume effects (economies of scale), particularly on module assembly cost. While RO has already travelled along the experience curve for a few decades, economies of scale improvement are yet to be experienced by CDI technology [1].
   
   
2. As highest specific surface area at low resistivity is a key contributor to increased CDI performance, improvements in the choice of suitable electrode materials are of key importance. Key requirements for CDI electrode materials and associated CDI performance are:
   - large ion-accessible specific surface areas (accessibility of electrode surface to ions)
   - high electrochemical stability over pH and voltage range, to avoid oxidation and to ensure system longevity
   - fast ion mobility within the micro- and macro-pores of the carbon electrode (e.g. inter-particle distance, intra-particle pore sizes, pore structure and thickness)
   - high electronic conductivity of electrodes (metal like electronic conductivity to ensure that the entire electrode is charged without major voltage gradients and to avoid larger energy dissipation and heating up of the electrode)
   - low contact resistance between the porous electrode and the current collector
   - good wetting behaviour (good hydrophilicity ensures participation of the entire pore volume in the CDI process)
   - low material cost
   - ease and scalability of material processing for higher volumes
   - abundant availability of the material and low CO₂ footprint for material purchase
   - high bio-inertness to avoid biofouling of CDI cell [1]
   
   
3. A particular challenge is the combination of high specific surface area with high ion mobility. The ultimate limit to the pore size is the bare ions size, e.g. 1.16 Angstrom for Na⁺ and 1.67 Angstrom for the Cl^-, or, more precisely, to 3.58 Angstrom for sodium and 3.31 Angstrom for the chloride as solvated ions [1]. Commonly the pore sizes for most common carbons will be significantly larger (in the small nm area).
   
   For reference: 1 Angstrom = 0.1 nm
   For further reference: IUPAC Classification of pore size:
   1. Macropores: > 50 nm
   2. Mesopores:  2 – 50 nm
   3. Micropores: < 2 nm [1]
   
   Carbon materials of relevance for ion adsorption in porous electrodes are amongst others:
   • activated carbon (100 – 3,500 m²/g)
   • ordered mesoporous carbon (OMC – typically 750 – 1,500 m²/g)
   • carbon aerogels (400 – 1,100 m²/g)
   • carbide-derived carbons (1,200 – 2,000 m²/g)
   • carbon nanotubes and graphene (fluctuating largely from 50 – 500 m²/g)
   • carbon black (< 120 m²/g) [1]
   
   
4. Improvement in operational modes to increase yield in deionised water (e.g. move to continuous/semi-continuous operation; recycle-loop layout) [1]
   
   
5. Improvement in cell unit design (e.g. wire type anodes) [15]
   
   
6. Further reduction of energy cost by harvesting the energy generated in the ion adsorbing (charging) CDI module
   
   
   
   
   
   

10. References

[1] Porada, S.; Zhao, R.; van der Wal, A.; Biesheuvel, P.M.: Review of the science and technology of water desalination by capacitive deionization. Progress in Material Science 58, (2013), pp. 1388-1442
[2] Anderson, M. A.; Cudero, A. K.; Palma, J.: Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practice: Will it compete? Electrochimica Acta 55 (2010), pp. 3845-3856
[3] Voltea: Voltea´s Technical Bulletin – Desalination Cooling Water with CapDI, URL: http://www.aquafit4use.eu/userdata/file/Mid-term%20Conference/Session%203%20Advanced%20Treatments/Voltea%92s%20desalination%20technology%20applied%20on%20cooling%20water%20B%26J.pdf (accessed 16.01.2017)
[4] Al Marzooqi, F. A.; Al Ghaferi, A. A.; Saadat, I.; Hilal, N.: Application of Capacitive Deionisation in water desalination: a review. Desalination 342 (2014), pp. 3-15
[5] Oren, Y.: Capacitive deionization (CDI) for desalination and water treatment – past, present and future (a review). Desalination 228 (2008), pp. 10-29
[6] Voltea: Voltea´s Technical Bulletin – Feed Water Guidelines, URL: http://voltea.com/wp-content/uploads/2016/03/402D021_Rev02-Technical-Bulletin_Water-Quality-Criteria.pdf (last accessed on 16.01.2017)
[7] Voltea: Technical Specification Industrial Series – IS 2 to IS 48 Systems, URL: http://voltea.com/wp-content/uploads/2016/03/402D026EN_Rev06-IS-Technical-Specifications.pdf (last accessed on 16.01.2017)
[8] Voltea: Voltea´s Technical Bulletin – Preservation Handling Guidelines, URL: http://voltea.com/wp-content/uploads/2016/03/402D004_Rev01_Tech-Bulletin_Preservation-Handling-Guidelines-1.pdf (last accessed on 16.01.2017)
[9] Welgemoed, T. J.: Capacitive Desalination Technology: Development and Evaluation of an Industrial Prototype System, University of Pretoria; Dissertation (2005)
[10] Voltea: Technical Comparison CapDI vs. RO, URL: http://voltea.com/wp-content/uploads/2016/05/CapDI-vs-RO.pdf (last accessed on 16.01.2017)
[11] Voltea: Voltea´s Technical Bulletin – Technology Comparison, URL: http://www.voltea.com/wp-content/uploads/2015/12/402D002_Rev01_Tech-Bulletin_Technology-Comparison-1.pdf (last accessed on 16.01.2017)
[12] Subramani, A.; Jacangelo, J. G.: Emerging desalination technologies for water treatment: A critical review. Water Research 75 (2015), pp. 164-187
[13] Atlantis Technologies Website, URL: http://www.atlantis-water.com/value-proposition/ (last accessed on 29.04.2018)
[14] Voltea Website, URL: http://voltea.com/technology-4/ (last accessed on 16.01.2017)
[15] Porada, S., Sales, B. B., Hamelers, H. V. M., Biesheuvel, M.: Water Desalination with Wires. Journal of Physical Chemistry Letter 3 (2012), pp. 1613-1618